1. Field of the Invention
The present invention generally relates to modulation systems where data are carried in the form of symbols by carriers.
2. Discussion of the Related Art
FIG. 1 shows a band 1 of the spectrum of a signal conventionally modulated by a modulation of QAM, BPSK, QPSK, MPSK, etc. type. Band 1 is centered on a frequency Fo corresponding to the carrier frequency. The total width of the band, B, extending on either side of central frequency Fo, substantially corresponds to the symbol frequency.
A signal received, for example, by a satellite receiver, exhibits many bands or channels of the type of band 1. Each band is associated with a specific symbol frequency, or with two frequencies if the carriers are in quadrature, and each carrier or pair of carriers is associated with a specific symbol frequency.
For the processing of this type of signal, it is necessary to precisely know the values of the carrier frequencies and of the symbol frequency.
A problem is posed, since the carrier frequencies and the symbol frequency are not always known by the receiver.
Determining these frequencies is difficult since it is a research on two parameters, which may vary within large proportions. For example, in a signal originating from a satellite receiver antenna, the carriers (or channels) are distributed over a range that can vary from approximately 1 to 2 GHz and, for each channel, the symbol frequency may for example range between 1 and some thirty or some forty Megabauds.
A conventional method to determine the carrier frequencies and those of the symbols consists of arbitrarily setting a symbol frequency, for example, 20 Mbauds, and of then scanning the entire spectrum for the carrier frequencies. After this, the symbol frequency is slightly modified, for example, it is changed to 20.1 Mbauds, and the spectrum scanning is started over again. This method is very long, even if, by a previous scanning, the general shape of the signal spectrum has been roughly determined.